1. Field of the Invention
This invention relates generally to controlling gain of the hydraulic pressure applied to the servos that engage and release transmission clutches and brakes.
2. Description of the Prior Art
Automatic transmissions that employ planetary gearing often use the same shift control element, i.e., friction brake or friction clutch, to produce gearshifts among multiple transmission gear ratios. Necessarily, the gain of the hydraulic pressure used to produce the torque capacity of each clutch or brake will be different depending on which gear is applied.
In order to achieve smooth gear shifting, hydraulic gain management is used to provide optimum torque capacity of the respective shift control element. For example, in six-speed transmissions, gain management for a given clutch is often achieved using the control pressure from a different clutch.
In automatic transmissions able to produce a greater number of forward gear ratios, this is difficult to achieve because each clutch is applied in a greater number of gears. A single clutch control pressure cannot provide enough logical information to allow for gain management in transmission having a large number of forward gears.
In conventional practice, solenoid and spool valves are controlled by a system wherein the output of the command solenoid is routed to the designated spool valve being controlled, as well as to a shift valve. The shift valve is used to selectively route solenoid pressure to an additional area on the spool valve, thereby increasing the area on which solenoid pressure acts, resulting in a change in valve gain, i.e., the output/signal pressure ratio. The shift valve in these systems is controlled by a solenoid dedicated to this purpose. Additional dual gain regulators would then require additional shift valves and shift valve control solenoids.